Getter for melt-grown scintillator ingot and method for growing the ingot

ABSTRACT

The addition of a getter consisting essentially of reactive oxides of boron and silicon, to a melt of an alkali metal halide serves to overcome problems of unacceptable color, afterglow and hardness attributable to trace impurities present in the melt. These trace impurities are generally metals present in a concentration less than 1 part per million (ppm) parts of melt. An ingot melt-grown from charge stock treated with the getter provides high quality optical bodies such as light pipes, laser windows and scintillators. Specific problems characteristic of a scintillator ingot grown from a highly purified alkali metal halide &#34;remelt&#34;, such as is obtained by crushing and melting portions of a melt-grown ingot, are overcome by adding to the remelt a portion of fresh powder stock in which the getter has been uniformly distributed. 
     A process is taught for the Stockbarger growth of a scintillator ingot from a charge stock treated to include a getter consisting essentially of the combined reactive oxides of boron and silicon, comprising superheating a melt of treated charge stock for a period of time sufficient to react at least some of the reactive oxides with deleterious trace impurities present in the melt, and growing a scintillator ingot free from discoloration, afterglow or undue hardness due to the presence of the impurities.

BACKGROUND OF THE INVENTION

It is well known that melt grown ingots such as are used in theproduction of commercially acceptable optical bodies, must pass the moststringent quality control requirements. In fact, the continuing demandfor better performance of optical bodies requires that ingots be grownas nearly perfectly as possible. Since impurities in the melt generallydegrade performance as an optical body, it is essential that growthstock for a melt-grown ingot be as pure as possible. By this is meantthat deleterious impurities in crystal stock be less than 1 part permillion (ppm). Even so, if often happens that stock which passes themost stringent purity specification still unpredictably yields an ingotwhich is unacceptable from the standpoint of color, afterglow orhardness.

By "unacceptable color" we refer to the photosensitivity of an ingot asevidenced by solarization. By "solarization" we refer to darkening ofthe crystal when it is exposed to light. Crystals which resistsolarization are deemed to have acceptable color, that is a water-whitecolor. In fact, an ingot may be removed from the crucible in which it isgrown, and appear to have perfectly acceptable color, yet, after it isexposed to ultraviolet light for a short period of time, the ingot isvisibly darkened. Such darkening of an ingot, though it may fade in timeat room temperature, indicates the presence of impurities which may bepresent in so small a concentration as to be undetectable by any of theconventional analytical means, whether chemical or physical. An ingotwith a color problem is scrapped. Since the cost of scrapping even arelatively small ingot is substantial, it is unnecessary to state thatone does everything possible to avoid the cost of scrapping an ingot inexcess of 20 inches in diameter, which cost can easily exceed severaltens of thousands of dollars.

By "unacceptable afterglow" we refer to a scintillation phosphor("scintillator") which upon excitation produces light pulses over aperiod substantially more than 10 microsecs, at a level of intensitywhich interferes with the measurement of light output of subsequentpulses. A low quality scintillator may exhibit an afterglow for manyseconds, and even minutes.

By "unacceptable hardness" we refer to a noticeable increase in hardnessof the clear portion of the ingot, due for example, to the distributionof calcium impurity, which makes a difference in the type of surfacefinish which may be imparted the usable portion of the ingot. The typeof finish alters the reflectance of a sanded, machined or burnished(say, with steel wool), surface of scintillator units fabricated fromthe ingot.

This invention does not purport to permit the substitution of relativelyimpure charge stock for the high-purity charge stock required in thegrowth of scintillator ingots, but it does provide a solution to theaforedescribed problems which unexpectedly arise even when high-purityor ultra-pure stock is used. A typical charge stock for a melt-growningot may be commercially available crystals of ultra-purity, or stockobtained by purification of commercially available, less pure material,as described for example in U.S. Pat. No. 2,640,755 to J. Hay. Suchvirgin crystalline charge stock typically used for growing ingots, isreferred to as "fresh powder stock". Charge stock may also constitutescrap obtained from cutting useful scintillators from an acceptableingot. Such scrap, after inspection to reject pieces having visibleinclusions of foreign material, is referred to as "remelt scrap" and isgenerally charged, after it is crushed, to a crucible. In some cases,large pieces of remelt scrap can be stacked in the crucible providedtheir shape does not allow the charge to shift during melt-down.

As is well known, the use of getters or scavengers in the growth ofmelt-grown ingots involves a mechanism particularly noted for itsunpredictability. Typically, a getter is used to remove a specificimpurity known to be present in a particular melt-grown ingot. Forexample, a trace of free bromine in the atmosphere above a melt of analkali metal chloride or alkali metal bromide, is disclosed in U.S. Pat.No. 4,055,457. The bromine serves to suppress sulfate, nitrate andnitrite ions. In another example, potassium chloride ingots are grown inthe presence of carbon tetrachloride in the atmosphere, which CCl₄ atgrowth temperature provides phosgene to scavenge oxygen (see Pastor, R.C. and Braunstein, M., Air Force Weapons Laboratory ReportAFWL-TR-72-152, Vol. II, p 103-108, July 1973).

As is also well-known, scintillator ingots are grown in fused silicacrucibles, particularly when water is excluded by an inert gas sweep, astaught by Lafever, R. A. in U.S. Pat. No. 2,984,626. It is possible thatthe melt is inadvertently contaminated with silica if the temperature ofthe melt is high enough, and/or the charge contains an impurity that isalkaline, or generates a basic reaction. Whether or not this silicacontaminant is deemed active, there is no evidence of the extent, ifany, of such possibly beneficial contamination. Ingots grown in fusedsilica crucibles from pure growth stock, in the absence of moisture andwith an inert gas sweep, are indistinguishable from those grown from thesame charge stock in platinum crucibles, all other conditions of growthbeing the same.

We have previously used silica (SiO₂) alone in a reactive form, as auseful getter for a melt from which a scintillator ingot is grown.However, silica alone, in an amount in the range from about 10 ppm toabout 100 ppm, produces an undesirably high amount of "floc". Thiscotton-like floc is unavoidably retained as inclusions in scintillatorunits fabricated from the ingot. More importantly, the use of activeSiO₂ alone requires a relatively high "soak temperature" at which themelt is superheated during the period before crystal growth to make theSiO₂ react with the deleterious trace impurities, and also to meltsilicates (disilicates) formed by the reaction with active silica alone.The soak temperature is generally greater than 100° C. above the meltingpoint of the charge stock. With addition of reactive oxides of boron(hereafter "borate" for brevity), the soak temperature is generally lessthan about 200° C., and preferably less than about 100° C. above themelting point of the charge stock. Borate addition, by itself, in theabsence of active silica, shows no appreciable change in performance orreduction in sensitivity.

Minor variations in a method of growing an ingot often result in asubstantially different optical body, whether it be for the better orfor the worse. At the very low concentrations of impurities which proveto be deleterious, contamination from the furnace becomes a surprisinglyimportant factor when growing an ingot from ultra-high purity stock.Heretofore, improvements in ingot quality were sought by minoralterations in the conditions of growth, and/or a beneficial, ifunpredictable, stratification of the ingot in such a way as toconcentrate the impurities in a portion of the ingot which can bediscarded. However, where concentration of deleterious impurities is solow as to be undetectable by conventional chemical or spectrographicmeans, it is most difficult to concentrate the impurity in anyparticular portion of the ingot, or to trace its origin. In other words,an effective getter must remove the effects of the deleteriousimpurities no matter what their origin or how they are distributed inthe ingot, which requires that the getter be able to combat the effectsof a wide range of contaminants.

Briefly stated, when metal or non-metal trace impurities are distributedthroughout a melt, an effective getter must: (1) be dispersed throughoutthe melt, and essentially homogeneouly distributed for Stockbargergrowth where melt-stirring is minimal: (2) react with the impuritiespresent without removing too much of a dopant or activator deliberatelyadded to the melt; (3) tie up the reaction products in such a way as notto affect the optical performance of the finished ingot; and yet (4)have characteristics such that the presence of the getter in themelt-grown ingot is not objectionable. These many exacting requirementsare satisfied to our knowledge, only by the combination of reactiveoxides of boron and silicon, (thus, referred to as a "combinationgetter"), or a compound which yields one or the other, or both, of thedesired reactive oxides. The oxides of titanium, aluminum, zirconium,lanthanum, gallium, tin, lead and other members of Groups III and IV ofthe Periodic Table, are ineffective getters, if not deleteriouscontaminants, in an alkali metal halide melt.

A melt-grown alkali metal halide ingot grown according to the process ofthis invention in which a combined getter of borate and active silicahas been used, has most of the floc formed by reaction of the getterwith impurities in the melt, distributed near either the upper surfaceof the ingot for some melts, or the bottom surface for other melts; and,some floc distributed around the sides of the ingot. Most of the floc isrejected by the melt, before or during growth of the ingot, in such away that the portions of the ingot containing most of the floc can bediscarded. The floc within the melt is characteristically presenttherein as metal silicate and metal borate solid particles or liquiddroplets which tend to cluster or agglomerate. Excess unreacted silicais insoluble in alkali metal halide melts, and collects along with themetal borosilicate floc. This floc tests high in boron and variousmetals not found in analysis of the growth stock, but the floc may ormay not contain all of the added borate component of the getter, some ofthe borate ends up in solid solution in the crystal when the proceduredescribed herein is followed. Such an ingot, grown from a treated melt,fails to exhibit visible darkening upon exposure to ultraviolet light inan amount which would darken an otherwise identical ingot which was notgrown from a melt which was treated (hence referred to herein as"treated melt" or "treated ingot"), with the combination getter.

SUMMARY OF THE INVENTION

It has been discovered that objectionable color, afterglow and hardnessin a melt grown scintillator ingot may be overcome by the addition of acombination of getter components consisting essentially of reactiveoxides of boron ("borate" for brevity), and silicon. Borate isexemplified by boric acid, or other compound with a B--O bond, such assodium tetraborate, sodium fluoborate, and the like, which compound isat least slightly soluble in the melt, and which also yields a reactiveboron oxide when the melt is superheated or "heat-soaked".

It has also been discovered that high quality laser windows may beproduced from a monovalent metal halide melt, and particularly an alkalihalide melt. These windows have a high breakdown threshold toradiation-induced damage greater than 6 J/cm². Breakdown threshold isthe damage caused by a high-power pulsed laser.

It is therefore a general object of this invention to provide anultrapure charge stock from which a high-quality alkali metal halideingot may be meltgrown, which charge stock is treated to include fromabout 5 ppm to about 1000 ppm of an active SiO₂ component, and fromabout 5 ppm to about 1000 ppm of a borate component, computed as BO₂ ⁻,which together comprise a combination two component getter.

It is also a general object of this invention to provide a process forgrowing a high-quality alkali metal halide ingot from a charge stocktreated with the aforementioned combination getter, comprisingheat-soaking the melt at a temperature in the range from about 10° C. toabout 150° C. above the melting point of the charge stock, for a periodof time sufficient to react at least some of each component withdeleterious impurities, and particularly, metal impurities in the melt.Typically deleterious impurities are present in the melt in traceamounts sufficient to yield an optical body with objectionable color,afterglow and hardness, which are characteristics when no getter isadded.

It has also been discovered that a melt-grown scintillator ingot havingobjectionable color, afterglow and hardness, may be remelted andregrown, if a portion of weight of "remelt" is combined with a portionof fresh powder stock to which is added a combination getter of borateand silica. An ingot grown from the remelt and fresh powder stock mixedwith getter has been found to be purged of objectionable characteristicsof an ingot grown from remelt alone, or re-melt from re-melt combinedwith fresh powder stock which has not been treated with the combinationgetter.

It is also a specific object of this invention to provide a process formelt-growing an ingot according to the Stockbarger method, from a chargestock containing a major amount of remelt scrap, and a minor amount offresh powder stock, or vice versa, by deliberately adding to thecrystalline charge stock, a combination getter consisting of slightlymelt-soluble borate and insoluble but active silica distributedthroughout the charge stock, and superheating the melt prior tocommencing growth, to a temperature lower than about 200° C. above themelting point of the charge stock, for a period of time sufficient toallow reaction of the combination getter with impurities present in thestock or derived from furnace components, for example, from the nickelalloys used in heating elements, or from crucible supports, and thelike.

It is another specific object of this invention to provide a process forgrowing an activated alkali metal halide scintillator characterized byreproducible water-white clarity and a freedom from undue hardness,objectionable afterglow or discoloration when exposed to ultravioletradiation, and which is further characterized by excellent resolution.

It is a more specific object of this invention to provide a water-whitesodium iodide:thallium (NaI:Tl) ingot having characteristic metaborateion absorption bands at about 5 and 17 microns, or a series ofunresolved band groups near 7, 8 and 13.5 microns for borates orpolyborates containing B--O--B bridge bonds, or both, resulting from amelt having a combination borate and active silica getter added theretoin an amount from about 5 ppm to about 1000 ppm borate, computed as BO₂⁻, and from about 5 ppm to about 1000 ppm silica, computed as SiO₂,which getter reacts with deleterious metal impurities, present in themelt in an amount less than 1 ppm. Such impurities commonly present arenickel, lead, iron, manganese and silver, none of which is present, byitself, in an amount greater than 1 ppm of melt.

It is a further specific object of this invention to provide a processfor melt-growing an undoped alkali metal halide light pipe, or lowtemperature scintillator ingot of ultrapurity. Such an ingot, forexample, of sodium iodide free of traces of thallium, is an excellentscintillator at or below about dry ice temperature, and at roomtemperature is characterized by very low relative pulse height, so it isused as a light pipe to shield a NaI(Tl) crystal from gamma rays emittedby the phototube.

Yet a further discovery incident to the use of the aforementionedcombination getter in a melt-grown alkali metal halide, is that thegetter reacts to form silicates and borates of deleterious impuritieswhich may include Group II A elements including alkaline earth metalssuch as calcium, strontium and barium, Group II B elements such as zinc,Group III A elements such as aluminum and thallium, Group III B elementssuch as the rare earth elements, elements of Group IV B such aszirconium, Elements of Group V B such as vanadium, elements of Group VIB such as chromium, elements of Group VII B such as manganese, andelements of Group VIII particularly iron and nickel, the presence of oneor more of which, though not all related to the problems of afterglowand discoloration, are likely to produce undue hardness.

It is therefore also a specific object of this invention to provide aprocess for growing an acceptable water-white scintillator ingotcharacterized by a freedom from objectionable hardness, whether grown byStockbarger or Kyropoulous methods.

These and other objects, features and advantages of the process of thisinvention, and the ingot formed by using the afore-identifiedcombination getter will become apparent to those skilled in the art fromthe following description of preferred forms thereof and the examplesset forth herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

As hereinabove stated, this invention is directed to the use of acombination getter in a melt of an alkali metal halide which iscontaminated with trace quantities of undesirable metal and non-metalimpurities. Discoloration is most unwanted in scintillation phosphorssuch as are used in scintillator detectors. By "scintillation phosphors"we refer to those phosphors which have very short decay times less thanabout 10 and preferably less than about 2 microseconds. The performanceof these phosphors is measured by the phosphors' capability to resolve apreselected monoenergetic gamma ray near room temperature.

Those skilled in the art will recognize that color is not necessarilydetrimental to the performance of all optical bodies, but that colorformation by the passage of light in a scintillation phosphor adverselyaffects the resolution of the crystal. This is critical in largecrystals where the path length, from point of emission to the detector,is long and variable for different points of origin within the crystal.Scintillation phosphors of commercial significance are those whichexhibit good resolution at room temperature or above, thoughscintillators which perform best below dry ice temperature haveespecialized application, for example, as light pipes used at roomtemperature. Therefore, further detailed description of this inventionwill be specifically directed to a scintillation phosphor which is meltgrown from an alkali metal halide, and still more particularly, to aningot grown from an alkali metal halide which has been activated by theaddition of an activator or dopand. Most preferred scintillators aresodium iodide doped with thallium iodide (Na:Tl), cesium iodide dopedwith thallium iodide (Cs:Tl), potassium iodide doped with thalliumiodide (KI:Tl), cesium iodide doped with sodium iodide (CsI:NaI), andthallium chloride doped with iodides of (i) thallium, (ii) thallium andberyllium, or (iv) other combinations of activator iodides, all of whichare referred to herein as (TlCl:metal iodides).

The level of doping (activator concentration) required for an alkalimetal halide scintillator depends upon the particular use of thescintillator. Factors affecting the choice of activator for ascintillator and the concentration of the activator, are well known tothose skilled in the art. In general, the level of activator is lessthan about 10 mole percent, and more preferably is in the range fromabout 1 ppm by weight, to about 1% of the melt, so as to give a uniformenough response while obtaining acceptable resolution. As is wellrecognized by those skilled in the art, the concentration of activatorin the charge to the crucible is generally in excess of that requiredfor activation of the melt-grown ingot. In the present invention, usingthe combination getter, the concentration of activator is present in anamount sufficiently in excess of that required for scintillation, topermit depletion of the activator by reaction with the getter.

It is essential to recognize that the borate component of the getter isprovided in a form in which a B--O bond of the compound can react withimpurities in the melt, and the silica component is provided in amelt-insoluble, but reactive form. It is immaterial whether eachcomponent of the getter is added to the charge stock crystalsseparately, or together. It is also immaterial whether the componentsare added as finely divided solids or in liquid form, though it will beapparent that it is more convenient and accurate to add lowconcentrations of each component as a liquid. For example, boric acidmay be added to hot charge stock crystals as an aqueous solution whichof course will cause water to be evaporated immediately upon contact.Alternatively, a solid, finely divided alkali metal borate or metaboricacid may be sprinkled onto the charge stock crystals. Any "borate" whichis dispersible in the charge stock crystals, and which provides areactive B--O bond will serve as a getter to complement the getteraction of active silica. Such a compound is the essential boratecomponent of the combined getter, and is also referred to herein as areactive borate.

The silica component of the getter is preferably added with the borateto the charge stock crystals in either liquid or solid form. Forexample, silicic acid may be added as an aqueous solution, water fromwhich will be vaporized upon contact with hot charge stock.Alternatively, an ingot may be grown in a crucible which has beenpreviously coated with ethylsilicate and then ignited to depositinsoluble but reactive SiO₂ on the walls of the crucible. Or, silicicacid available as Cabosil* from Cabot Chemical Co. may be used to coatpowder crystals uniformly, and thereafter, the silicic acid may beheated to convert the silicic acid to silica. In other words, whateverthe manner in which the silica component is introduced into the melt, itis essential that it be present in the melt in its insoluble form, butcapable of reacting with metal impurities to form insoluble silicates.

Despite the demand for ultra-pure growth stock for the production ofalkali metal halide phosphors, growth stock sometimes inadvertentlycontains insoluble SiO₂ as an impurity. The presence of this impurity isgenerally less than about 1 ppm, though on occasion, stock with silicaimpurity approaching 2 ppm has been used. The presence of such a lowconcentration of silica impurity can be detected only with greatdifficulty by searching for and finding small inclusions of floc withinthe crystal. We are unaware of any growth stock for scintillator ingotswhich might have been fortuitously sufficiently contaminated with boththe silica component and the borate component, each in appropriateconcentration, so as to produce a scintillator ingot free ofdiscoloration, undue hardness or afterglow.

An alkali metali halide charge stock, and particularly the halides oflithium, sodium, potassium and cesium, of this invention is one whichtypically contains certain impurities but is treated with thecombination getter. Such impurities commonly are aluminum, calcium,magnesium, lead, and potassium (because of K⁴⁰ content), albeit in lowconcentration, each less than about 1 ppm of charge stock. Even loweramounts, less than about 0.5 ppm of copper and iron may also be present.With the combination getter, acceptable crystals may be cut from ingotsgrown from charge stock contaminated with relatively high levels of theforegoing impurities, which charge stock would not otherwise yieldacceptable crystals. These relatively high levels which may be toleratedare set forth in a typical analysis of charge stock of this invention inTable I hereinbelow, under the heading "Max. ppm", correspondingapproximately to levels detected by emission spectroscopy which arelisted under the heading "Max emission spec level".

                  TABLE I                                                         ______________________________________                                                Typical conc                 Max                                      Component                                                                             (ppm)      Max emission spec level                                                                         ppm                                      ______________________________________                                        Al      less than 1                                                                              FT.sup.- (faint trace minus)                                                                    5                                        Ca      less than 1                                                                              VFT(very faint trace)                                                                           2                                        Mg      less than 0.5                                                                            VFT.sup.- (v faint trace minus)                                                                 1.5                                      Cu      less than 0.5                                                                            EFT.sup.- (ex faint trace minus)                                                                0.5                                      Fe      less than 0.5                                                                            EFT.sup.+ (ex faint trace plus)                                                                 1.0                                      Pb      about 0.015                  0.04                                     K       less than 0.5                1.2                                      SiO.sub.2                                                                             13         9 ppm (min)       17.                                      H.sub.3 BO.sub.3                                                                      10         6 ppm (min)       14.                                      ______________________________________                                    

The following impurities may sometimes be present, but are usuallypresent in a concentration so low as not to be detectable by emissionspectroscopy: arsenic, bismuth, beryllium, cadmium, cerium, chromium,cobalt, gallium, germanium, indium, sodium, nickel, molybdenum, silver,strontium, titanium, tungsten, vanadium, zinc, and zirconium.

In general, about equal parts by weight of the borate and silicic acidare thoroughly mixed together, and uniformly dispersed throughout thecrystals of charge stock in a crucible in which a Stockbarger ingot isto be grown. It is preferred to use less than 1000 ppm of eachcomponent, computed on the basis of active SiO₂ BO₂ ⁻, though there isno particular upper limit as long as the excess is rejected from theingot. It is most preferred to use from about 10 ppm to about 50 ppm ofeach component. The ratio of the borate and silica components may bevaried depending upon the sensitivity of the melt to superheating. Themore borate component in the getter, the lower is the temperature atwhich the getter is reactive. The charge stock crystals carrying thecombination getter on their surfaces are melted and then graduallysuperheated. By superheating we refer to the step of heating the moltencontents of a crucible to a temperature less than about 200° C. above,and preferably in the range from about 50° C. to about 100° C. above themelting point of the alkali metal halide for a period of time in therange from about 30 minutes to about 15 hours depending in some measureon the size of the ingot.

Those skilled in the art will recognize that it is desirable to avoidthermal stirring set up by convection currents in a Stockbarger melt.The higher the superheating temperature the greater the thermal stirringeffect, so it is desirable to have a low superheating temperature, foras short a period as possible, without overloading the melt with theborate component of the getter.

Though it will be apparent that sufficient combination getter isdesirably added to react with all the impurities responsible forobjectionable color, afterglow and hardness in a crystal, excessiveaddition of the getter must be avoided. A too-long excess may not berejected sufficiently to one or the other extremities of an ingot, whichextremities can be conveniently discarded. An amount of either componentin excess of about 200 ppm of melt is excessive and generally results inthe formation of floc within the crystal which interferes with theperformance of the crystal particularly when it is used as a cameraplate. Such accumulations of floc might lead to a mistaken diagnosiswhen a scintillation phosphor is used for a camera plate, as forexample, in an Anger camera used for the detection of tumors.

Though the presence of from about 10 ppm to about 50 ppm of silica andborate, each, might counter the color problem in fresh powder stock, thepresence of the same, or even a larger amount of residual borate orsilica in remelt scrap shows no beneficial effect, because both excessresidual silica and borate in remelt scrap are in an inactive form.Addition of combination getter to remelt scrap alone improves thequality of an ingot grown, but the ingot is not quite as good as that ofan ingot grown from remelt scrap containing a major portion by weight offresh powder stock, or from fresh powder stock alone. Thus, thoughremelt stock is purer than fresh powder stock, it is preferably used inconjunction with a major amount of fresh stock to which silica andborate have been deliberately added, and uniformly distributed.

In a particular embodiment this invention is of particular value formelt-growing an ingot from high purity sodium iodide fresh powder stockcrystals which contain trace color-forming metals such as nickel, lead,iron, manganese, silver and the like, or impurities which interact withthallium activator to increase the after-glow of the melt-grown ingotand increase the tendency of the ingot to solarize.

By using charge stock treated with the combination getter, the processof this invention permits the Stockbarger growth of a large ingot inwhich unwanted impurities are rejected towards the upper and lowerportions of the ingot, and also to its sides. Thus only a minimalportion of the ingot is discarded. The result is that a nearly"full-size" ingot is recovered. In other words, very little of the ingotmust be discarded. Because impurities which affect resolution have beenremoved, the acceptable ingot characteristically has predictableperformance. Minimum labor is then expended temporarily to mount andtest portions of the ingot for different surface preparations to attaina specified resolution for each portion. It will be apparent thatimpurities which harden the crystal will make it more difficult toprepare the surface. Less apparent is that, because the impurities whichnormally harden the ingot have been removed, the ingot can bepress-forged or extruded with less difficulty than that encountered witha prior art ingot.

An alkali metal halide ingot grown from treated charge stockcharacteristically has metaborate absorption bands at about 5 and 17microns, or a series of unresolved band groups near 7, 8 and 13.5microns for borates or polyborates containing B--O--B bridge bonds, orall the absorption bands. The presence of these absorption bands in thecrystal correlates with low afterglow for 3600 A light, and indicatesthat the crystal was treated with the combination getter. Thecorrelation is not proportional, but an absorption of at least 10% perinch (or 0.04 cm⁻¹) at about 5 microns is a "fingerprint" characteristicof a treated ingot which exhibits low afterglow. This fingerprint alsoindicates that the crystal was obtained from an ingot grown from atreated melt which was heat-soaked at an appropriate, not-too-high heatsoak temperature.

If the heat soak temperature is too high, that is greater than about150° C. over the melting point of the charge stock, the absorption of5.15 microns for BO₂ ⁻ is about 1% per inch, or less. Typically, such aningot which is over-heat-soaked has marginally acceptable afterglow, butsatisfactory photosensitivity and hardness.

The following Example 1 illustrates a typical procedure for melt-growinga conventional 10 inch NaI(Tl) ingot in a Stockbarger furnace from acharge of ultra-pure fresh powder stock, utilizing the method generallydescribed in U.S. Pat. No. 2,149,076 the disclosure of which, modifiedto obtain a controlled atmosphere, is incorporated herein. Thismodification comprises (1) evacuating the furnace and charge, and (2)admitting an inert atmosphere to control evaporation during growth.

EXAMPLE 1

39.7 kg of fresh powder sodium iodide stock is loaded into a 10"diameter platinum crucible and 80 g of thallium iodide crystals areadded to it. The crucible is loaded into a Stockbarger furnace forgrowth using a slow elevator speed. The ingot is melted out of thecrucible and annealed. It showed excellent color and very fewaccumulations of floc. However, when the ingot was exposed to a 30 wattultra-violet lamp, emitting a 3600 Angstrom wavelength, for a period ofone minute, the ingot darkened visibly. This indication of darkening isindicative of an unacceptable ingot.

It is noted that the ingot has objectionable hardness evidenced by thequick dulling of cutting tools used to dress and section the ingot. Thisobjectionable hardness was also evidenced when a portion of the ingotwas press-forged into a polycrystalline body at a temperature below itsmelting point and sufficient pressure to effect the crystallinetransformation as described more fully in U.S. Pat. No. 4,063,255. Thepress-forging was unsatisfactory.

A sodium iodide:thallium NaI(Tl) scintillator ingot was melt-grown asdetailed in the following Example 2, utilizing a combination getter,using Stockbarger-type growth. It will be recognized that, though it maybe less desirable because of the precautions necessitated, a NaI(Tl)ingot, with the combination getter deliberately added to the melt, maybe grown by Kyropoulos growth.

EXAMPLE 2

39.7 kg of fresh powder stock sodium iodide crystals was weighed into alarge mixing vessel heated to keep the crystals at a temperature aboveabout 80° C., and preferably at a temperature in the range from about120° C. to about 150° C.

A solution of 2.46 g Na₂ SiO₃.9H₂ O and 0.4 g H₃ BO₃ in 100 ml water isacidified with 50% HI and sprayed onto the crystals which are heatedabove a temperature which will allow NaI.2H₂ O to form. The crystalscontaining about 10 ppm H₃ BO₃ and 13 ppm SiO₂ are placed in a 10 inchplatinum crucible along with 80 g TlI crystals. An ingot is then grownin a Stockbarger furnace under an atmosphere of nitrogen.

After melt-down the melt is given a heat soak at elevated temperature,that is, superheated gradually until the melt in the cone of thecrucible is about 100° C. above the melting point. The period of heatsoaking is about 4 hr from the time the charge is melted until thetemperature is set down for growth of the ingot. The ingot grown ismelted out and annealed. It has excellent color and remained water whiteafter exposure for one minute to the ultra-violet light used in theprevious example. It is free from objectionable afterglow and hardness.It is easily cut without dulling the cutting tools and it ispress-forged under the same conditions which failed to produce apress-forged optical body in Example 1 hereinabove.

The ingot melt-grown from a treated melt displays narrow bands at about5 and 17 microns for metaborate ions identified as disclosed in J. Appl.Phys. Supp. 33 (1) 364-366, 1962 by Morgan H. W. and Staats, P. A.; or,a series of unresolved band groups near 7, 8 and 13.5 microns forborates containing B--O--B bridge bonds, comparable to modifications ofabsorptions found in the vapor of H₂ B₂ O₃, as disclosed in Inorg. Chem.8 (4) 731-7, 1969 by Grimm, F. A. and Porter, R. F.; and sometimes, theingot displays both the narrow bands and the unresolved band groups. Theunresolved band groups are attributable to polyborates which includecondensed borates, B₄ O₇ ⁻², fluorooxyborates B₄ F₁₂ O⁻², as well aspolymetaborate (BO₂)_(n) ^(-n) rings of variable length.

The infrared spectra for a crystal from near the cone of the ingotshowed a series of very narrow bands for BO₂ ⁻ ions at 3.23, 3.26, 3.43,5.01, 5.18, 5.21, 6.58, 6.77, 16.7, 17.4 and 20.1 microns. The strongestband is at 5.18 microns being 21% per inch of crystal. Crystal from nearthe heel of the ingot showed the 5.18 microns bands at 30% per inch, andalso showed bands for unresolved groups for (BO₂)_(n) ^(-n) as follows:group I--6.78, 6.83, 7.0, 7.18; group II--7.87, 7.92, 8.05, 8.42; groupIII--13.9, 14.2; group IV--9.3; group V--10.5; group VI--18-20; groupVII--5.78; and, group VIII--6.13, 6.36. Group I was the strongestshowing an absorption of 17.5% per inch of crystal.

EXAMPLE 3

The previous examples 1 and 2 utilized fresh stock powder and no scrapcrystals. In this example 20 kg of scrap crystals of sodium iodidethallium, after visual inspection for discontinuous, visible inclusionsand a wash of the surfaces, were dried and crushed for remelting andcharged to a 10" diameter platinum crucible. To this charge was added 20kg of fresh stock powder sodium iodide crystals containing about 20 ppmH₃ BO₃ and 20 ppm SiO₂ added in the same way as in the previous example,namely by spraying an acidified aqueous solution of H₃ BO₃ and silicaacid on the heated crystals. 70 g thallium iodide crystals are placed inthe crucible with the NaI crystals. The charge was melted down in theplatinum crucible under approximately one atmosphere of nitrogen,superheated in a manner identical with that described in Example 2hereinabove, and melt-grown under one atmosphere of nitrogen. The ingotwas melted out from the crucible and annealed. It showed excellentwater-white clarity. The ingot was exposed to the 30 watt UV fluorescentlamp for a period of one minute, as were the ingots grown in Examples 1and 2 hereinbefore. No visible darkening of the ingot was observed. Thisindicated that the scintillator had acceptable color, and that theunacceptable color characteristic of an ingot grown from too-pure remeltscrap had been effectively controlled.

EXAMPLE 4

In a manner analogous to that described hereinabove in Example 3, 50percent remelt scrap crystals was used in which floc accumulations ofsilica were present. Fresh stock powder was added in an amount of about50 percent by weight, except that no silica was added with the freshstock. The ingot which was given a heat soak as in Examples 2 and 3hereinabove, was melt-grown in the Stockbarger furnace. The ingot showedgood color when removed from the annealing furnace at room temperature,but visibly darkened when exposed to the UV lamp for a period of oneminute. This indicated that prior existing silica in remelt scrap ispresent in a form which is not active, that is, will not react to formmetal impurity silicates, and therefore is ineffective to provide thenecessary getter action to counteract the color problem in the scrap.

EXAMPLE 5

A sodium iodide melt-grown ingot is grown, in a furnace in which nothallium doped crystal has been grown. Commercially available, puresodium iodide fresh stock powder is used having less than 0.5 ppmpotassium, no measurable thallium impurity, and to which 10 ppm H₃ BO₃and 15 ppm SiO₂, measured as SiO₂, is added. The melt was given a heatsoak, that is, superheated for 2 hrs at 735° C. The melt-grown ingot,after annealing, showed excellent water white color. Exposure to the UVlamp for a period of one minute produced no visible darkening of theingot. Samples 1" diameter ×1" high from the ingot, have a relativepulse height (RPH) at room temperature of less than 1% of NaI(Tl) wherea soda-lime glass window is used. Crystals from the same furnace, withthe same fresh powder stock and with no combination getter added, showRPH in the range from about 1.5% to about 2% of NaI(Tl). This differenceallows the low energy sensitivity of an instrument to be improved from0.03 MEV down to 0.015 MEV when the grown crystal is used as a lightpipe for a NaI(Tl) scintillator. In this application Tl as low as 0.02ppm contributes to pulse height, along with other elements such as tin,indium, cadmium, etc. which may be present at very low concentrations,and which nevertheless contribute to pulse height. Even such traceimpurities can be removed by the combination getter of borate andsilica. The crystal shows from about 10% to about 40% per inch,absorption, at 5.15 microns for BO₂ ⁻ ions in solid solution, whichindicates than a melt-soluble borate was present. However, theseabsorptions do not detract from the use of a crystal as a light pipe forlight emitted from a NaI(Tl) scintillator.

EXAMPLE 6

A 21/4 inch diameter platinum crucible is loaded with 520 g of purecesium iodide and 2.25 g of sodium iodide (0.75 mole percent of addedsodium iodide) into which about 15 parts HBO₂ and 20 parts Cabosil*active SiO₂ per million parts CsI were blended as a dry mixture in a dryroom. The crucible is placed into a controlled atmosphere, Stockbargertype furnace having upper and lower chambers with an openingtherebetween and an elevator for supporting the crucible, and operableto move the crucible between the chambers. The crucible is mounted onthe elevator with the lower extremity of the crucible at the openingbetween chambers and extending into the upper chamber.

The furnace is evacuated at room temperature to a pressure of one-halfmicron. It is then heated to a temperature of 200° C. and held at thistemperature for a period of 13 hours during which time the evacuation byvacuum pump is continued and the pressure reduced to one-tenth micron atthe end of this period. The temperature of the furnace is then raised to400° C. and maintained at this temperature for 23 hours during whichtime a pressure of one-tenth micron is maintained. At the end of thisperiod, the furance is filled with helium gas to a pressure of oneatmosphere and the furnace temperature raised to 750° C. at the controlthermocouple and maintained for 6 hours, melting the charge in thecrucible. The contents of the entire crucible are then superheated, thetemperature near the top of the melt being about 850° C., and near thebottom about 750° C. The temperature of the upper chamber is thenlowered to 700° C. and maintained while the temperature of the lowerchamber is maintained at 460° C. The crucible is then lowered by theelevator at a rate of 1.4 millimeter per hour. After a growing time ofapproximately 50 hours, the crystal is removed from the furnace andmelted out of the crucible by heating the crucible to the melting pointof the crystal material for a brief period. The crystal is then annealedby lowering its temperature to room temperature at a rate ofapproximately 25° C. per hour. A crystal so prepared is water-white,that is free of color, and has better performance because thalliumimpurity is reduced to less than 1 ppm. The ingot exhibited pulseheights of 93% of NaI(Tl) and resolution of 8.8% when excited by gammaradiation from a cesium 137 source.

EXAMPLE 7

A paltinum crucible in which an ingot has been grown by a conventionalKyropoulos procedure, is allowed to cool until the surface of the meltbegins to solidify. Then 20 kg of fresh powder stock NaI crystals and 60g of TlI crystals are distributed over the surface of the solidifiedsurface of the melt. A mixture of 0.4 g Cabosil* silica and 0.28 gmetaboric acid powder is sprinkled into the charge. The temperature isthen gradually increased and the charge stock crystals with silica andborate getter are stirred into the melt. The melt is held at about 50°C. above its melting point for about 2 hours, or until the meltclarifies. Superheating in the range from about 10° C. to about 100° C.is generally effective, and a temperature in excess of 150° C. above themelting point of the charge is to be avoided. Floc formed by reaction ofthe combination getter with impurities in the melt, is rejected by themelt and moves to the walls of the crucible where most of the floc iscaught. When the melt clarifies, it indicates that the temperature maybe dropped, and the seed lowered into the melt to commence growth in theconventional manner. A boule grown from treated melt is remarkably freefrom afterglow, unacceptable color and hardness.

I now refer to a laser window produced from a sodium chloride orpotassium chloride ingot melt-grown from charge stock treated with thecombination getter. A laser window so produced may be cut from a largeingot, and therefore is essentially a single crystal; or, the window maybe produced by press-forging the ingot to form a polycrystallineoptically integral window, as described in U.S. Pat. No. 3,933,970, thedisclosure of which is incorporated by reference thereto as if fully setforth herein. When such a laser window is subjected to a pulsed laser ofhigh enough power, the window is cracked or otherwise damaged. Thoseskilled in the art will appreciate that a high breakdown threshold in awindow may be more desirable than higher strength and lower absorptionin a similar window with a lower breakdown threshold.

Testing of laser windows for breakdown thresholds when the windows aresubjected to 10.6 micron nanosecond pulses, is described in"Radiation-Induced Damage to Polycrystalline KCl and NaCl by 10.6. mNanosecond Pulses," by Reichelt, W. H. and Stark, E. E.;AFCRL-TR-74-0085 (111), Special Report No. 174, Los Alamos ScientificLaboratory, New Mexico. Prior art laser windows grown from alkali metalhalides, and particularly sodium chloride, sodium iodide, potassiumchloride and cesium iodide, had breakdown thresholds lower than 6 J/cm²,whether the window was essentially single crystal or not (that is,polycrystalline). Laser windows produced from ingots grown as describedin the following example 8, from a charge stock treated with thecombination getter exhibit breakdown thresholds greater than 6 J/cm².

EXAMPLE 8

Sodium chloride crystals, purified as described in U.S. Pat. No.2,640,755, while still damp with mother liquor, are blended with enoughmetso sodium metasilicate solution previously acidified with muriaticacid, to add about 130 parts per million parts by weight (ppm) SiO₂equivalent. The crystals are then dried, sampled and analyzed for silicacontent batch by batch.

50 kg of this analyzed growth stock which averages from about 110 toabout 120 ppm SiO₂, are slowly charged into a 14" (in) platinum cruciblewhile adding dropwise 16 ml of boric acid solution containing 1.0 g H₃BO₃ so as to distribute 20 ppm H₃ BO₃ in the charge.

The crucible is loaded into a Stockbarger type furnace and heated to anelevated temperature 115° C. above the temperature control setting whichis known to allow solid to form in the conical tip of a salt ingot ofthe same size previously grown in the same furnace. This elevated soaktemperature is held for 12 hr, then the control is lowered 115° C. Aftergrowth of an ingot, the crucible is removed and annealed to bring thecrystal ingot to room temperature intact.

This ingot 14" diameter by 7" high showed no haze or color in a Tyndallbeam of light and a very low level of scatter from inclusions oncomponent boundaries. All of the silica and most of the borate settledinto the cone or adhered to the crucible wall so it did not adverselyaffect the yield of usable crystal.

Transmission spectra show imfrared absorptions for polyborate in clearsamples from this ingot. These bands fall in three groups making broadbands with multiple peaks. Group I: 6.88, 7.0, 7.1, 7.2 microns; GroupII: 7.86, 8.08, 8.23 microns; Group III: 13.28, 13.47, 13.73, 13.9microns. For this ingot, the strongest absorption was in Group I, givingan absorption coefficient β=0.0085 cm⁻¹ in the cone and 0.194 cm⁻¹ inthe heel for (BO₂)_(n) ^(-n). The cone also had 0.008 cm⁻¹ at 5.02microns for BO₂ ⁻.

Since this crystal was intended for use in the infrared rather than inthe UV and visible (as in previous examples) the borate in solidsolution was held down by using a long soak and minimal scavenging withH₃ BO₃. Thus the borate absorptions did not restrict use of the crystalfor 10.6 micron laser windows. These borate absorptions would not permituse of the crystal for infrared spectrometer optics. Polished samples1.5" diameter by 0.4" thick were measured for total absorption using aCO₂ laser calorimeter. Here the coefficient β was 0.0010 cm⁻¹ in thecone and 0.0020 cm⁻¹ in the heel. This compared favorably with 0.0013cm⁻¹ which is the median value for routine testing of cone samples ofNaCl ingots to which no borate was added.

The ingot was cut into slabs for laser windows and, two pieces 3"diameter by 1.09 cm thick, were given a commercial laser window polish.Samples from high and low in the ingot were tested at several places forradiation induced damage by nanosecond pulses of 10.6 micron energy,followed by transmission spectra to show the amount of polyborateabsorption.

Damage appeared at 7.28+0.93 J/cm² in the sample with 0.021 cm⁻¹polyborate absorption and 8.20+0.15 J/cm² for the sample closer to theheel where the polyborate absorption was 0.051 cm⁻¹.

These breakdown threshold values are 40 to 80% better than those forNaCl crystal made from the same quality growth stock having activesilica scavenger but without any borate addition. Prior art NaClcrystals test 4.5 or 5 J/cm². From theoretical considerations theimprovment is thought to result from a reduction in multivalent cationimpurity.

Measurement of borate absorption (usually BO₂ ⁻ at 5.02 micron) in priorart NaCl ingots to which no borate was added, shows occasionalabsorption at very low levels (less than 0.001 cm⁻¹) and detectable infewer that one in ten ingots. This accidental presence of boron impurityis not enough to modify the silicate and provide a combination getteraction to remove multivalent cations from a melted halide of amonovalent metal.

In the treatment of monovalent metal halide charge stocks describedabove, most of the borate goes into the insoluble silica-silicate flocwhere it can be found by analysis. Also the infrared spectra between 8and 15 microns for these floc inclusions is altered. Instead of broadbands for Si₂ O₅.sup.═, Si₄ O₉.sup.═, SiO₂, etc., the addition of boratebreaks each silicate band into a series without a basic shift inwavelength. Even though phase data indicates Na₂ Si₂ O₅ and SiO₂ assolid phases for the system Na₂ O--SiO₂ --B₂ O₃, from X-ray and meltingpoints this infrared spectral change points to incorporation of boratein the silicate structure. Such a change in molecular structure andexposed groups for chains and rings could account for the improvedgetter action of borate-silicate over silicate alone and why it requiresmore than accidental trace boron impurity to modify the amount of silicarequired. Since the solubility of SiO₂ in these melts is virtually zerothe scavenger action requires transfer of multivalent cations from themelt into the dispersed solid silicate or borate-silicate. This requiresa substantial surface area of silica and sets its minimum amount thatis, in excess of 5 ppm, for operating within a practical time interval.

Chemically and physically the action of this borate-silicate getter isthe same for all monovalent metal halide salts. In an analogous mannerto that described for NaCl, other optical bodies of AgCl, AgBr, LiF,NaBr, NaF, KCl, KBr, KI, CsBr, CsI, RbCl, RbBr, TlCl, KRS-5, and KRS-6(see "Handbook of Electronic Materials" by Moses, A. J., Vol 1, p 90,IFI-Plenum Publishing Company, New York, N.Y. 1971) may be prepared forapplications in which the borate absorptions of the body are notdetrimental. In particular, a prior art silver chloride crystal used torecord and define the path of high energy particles may now be improved.Such a prior art silver chloride crystal is well-known to be affected byimpurities which heretofore have eluded analysis.

I claim:
 1. In a Stockbarger or Kyropoulos process for growing an alkalimetal halide scintillator ingot from a melt in a furnace, theimprovement comprising distributing in said melt from about 5 to about1000 parts per million parts (ppm) by weight of said melt, of eachcomponent of a combination getter consisting essentially of at least aslightly melt-soluble reactive oxide of boron as one component, and aninsoluble but active silicon dioxide as the other component, andsuperheating said melt for a period of time sufficient to react at leastsome of each component with trace impurities present in said melt. 2.The process of claim 1 wherein said melt consists essentially of aportion of melt-grown scrap crystals and a portion of fresh alkali metalpowder stock in which said combination getter is distributed.
 3. Theprocess of claim 1 wherein said reactive oxide of boron is present in anamount in the range from about 5 to about 100 ppm of melt, and saidactive silicon dioxide is present in an amount in the range from about 5to about 100 ppm of melt.
 4. The process of claim 1 wherein said alkalimetal is selected from the group consisting of lithium, sodium,potassium, cesium and rubidium.
 5. The process of claim 4 wherein saidalkali metal halide scintillator ingot is activated with an activatorselected from thallium iodide, beryllium iodide, and sodium iodide. 6.The process of claim 1 wherein said trace impurities are each present inamount less than about 1 ppm, and at least one impurity is a compound ofan element selected from the group consisting of Groups II, A, III A, IVB, V B, VI B, VII B, and VIII of the Periodic Table.
 7. The process ofclaim 1 wherein said ingot is grown by the Stockbarger method in acone-bottom crucible, and superheating comprises heating said melt to atemperature in the range from about 10° C. to about 150° C. above themelting point of material in the cone.
 8. The process of claim 1 whereinsaid ingot is grown by the Kyropoulos method, and superheating comprisesheating said melt to a temperature in the range from about 10° C. toabout 100° C. above the melting point of material in the crucible.